1. Field of the Invention
This invention relates to a cognitive radio system and method for wireless communication, and more particularly for searching clear and idle channels in Wireless Local Area Network (WLAN) by comparing an average value of Received Signal Strength Indicator (RSSI) in duration of RSSI event, which is distinguished from duration of WLAN event, wherein the WLAN channels include channels in 2.4 GHz Industrial, Scientific and Medical band (ISM band) and Unlicensed National Information Infrastructure band (UNII band).
2. Description of the Prior Art
WLAN is a network for wireless communication between any access point and its surrounding local area network and popularly used in office, home and public region. It follows the IEEE802.11 standard, and her extended versions IEEE802.11a, IEEE802.11b, IEEE802.11g, and etc. for different needs. WLAN communication keeps growing not only in developed connection between laptop PC and desktop PC, but also in developing connections of wireless stick and game controller, wireless home electronics (e.g. home audio and theater, and printer), and mobile phone. With more and more WLAN applications and other radio frequency (RF) devices, RF environment in WLAN region becomes more and more crowded.
The present working frequency bands for IEEE802.11 WLAN are 2.4 GHz (802.11b and 802.11g) which is public Industrial, Scientific and Medical band, ISM band and 5 GHz (802.11a). However, the other wireless communications such as Bluetooth and HomeRF also work on the same frequency band (ISM band) with WLAN and decrease rate of WLAN communication. Besides, some RF devices surrounding in home or working space such as microwave oven, wireless phone or some standby industrial devices can cause non-negligible interference to WLAN communication.
Increasing WLAN applications, Bluetooth and HomeRF communications, and the other RF devices make a crowded WLAN band. In the crowded band, WLAN communication between different access points (APs) becomes not only intermittent but also unrecognizable. In fact, it becomes harder and harder for a traditional WLAN device, which just passes WLAN packet during breaks of a busy band, to deal with the more crowded communication band. In a crowded RF environment (e.g. multi-AP consisting of computers and printers, wireless phone and microwave oven), breaks for WLAN packet transmission become fewer. Traditional WLAN device with single transmission way can not keep a usual transmission bandwidth and event can not work normally.